Devices that can treat tissue non-invasively are extensively used to treat numerous diverse skin conditions. Among other uses, non-invasive energy delivery devices may be used to tighten loose skin to make a patient appear younger, remove wrinkles and fine lines, contour the skin, remove skin spots or hair, or kill bacteria. Such non-invasive energy delivery devices emit electromagnetic energy in different regions of the electromagnetic spectrum for tissue treatment. High frequency treatment devices, such as RF-based devices, may be used to treat skin tissue non-ablatively and non-invasively by passing high frequency energy through a surface of the skin, while actively cooling the skin to prevent damage to a skin epidermis layer.
Modern high frequency skin treatment apparatuses employ multiple discrete temperature sensors whose sensor packages are mounted on and attached to an electrode assembly for ostensively monitoring the temperature of the treatment tip of the high frequency device. Conventional high frequency capacitive electrodes consist of a pattern of metallic features carried on a flexible electrically insulating substrate, such as a thin film of polyimide. Despite being separated from the skin by the intervening insulating substrate, the temperature readings of the treatment tip measured by the thermistors is representative of the actual skin temperature. The insulating substrate is a poor conductor of heat.
The non-patient side of the electrode in the electrode assembly in the treatment tip, on which the thermistors are conventionally situated, may be sprayed with a coolant or cryogenic composition under feedback control of the thermistors for cooling the skin contacting the electrode assembly. The controller triggers the cryogenic composition based upon an evaluation of the temperature readings from the thermistors. The temperature readings from the thermistors are dependent upon, among other factors, the spray pattern of the cryogenic composition, any pooling of the cryogenic composition near or over the thermistor, and the evaporation rate of any cryogenic composition wetting the thermistor, for example.
Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and/or mixtures containing these compounds have conventionally been used as the cryogenic composition. However, because of their high chlorine content, chemical stability, and long atmospheric lifetimes, such compounds, when released to the atmosphere, can migrate to the stratosphere where they undergo photolysis and deplete the earth's protective ozone layer. CFCs particularly contribute to depletion of the ozone layer, with the HCFCs depleting the ozone layer to a lesser extent. As a result, production of CFCs and HCFCs has been and continues to be severely limited and is scheduled for phase out in many industrialized and non-industrialized countries.
Accordingly, the industry is continually seeking new fluorocarbon based mixtures that offer alternatives, and are considered environmentally safer substitutes for CFCs and HCFCs. Of particular interest are mixtures containing hydrofluorocarbons that are nonflammable, non-toxic and environmentally benign in having zero ozone depletion potentials, low global warming potentials (less than about 150) and negligible atmospheric and terrestrial environmental impacts. One currently used cryogenic composition is 1,1,1,2-tetrafluoroethane (R-134a). However, this cryogenic composition has significant and undesirable global warming potential (GWP=1410).
Accordingly, a need exists to use cryogenic compositions that can be used as efficient and economical substitutes for CFC and/or HCFCs. A need still exists for developing cryogenic compositions with a particular combination of properties for more specific applications. For example, suitable replacements for CFCs, HCFCs and/or hydrofluorocarbons (HFCs) must be non-flammable, non-toxic, and unreactive and provide a desirable or low global warming potential, i.e., a GWP of less than about 150. Further, in order for a cryogenic composition to adequately serve as a replacement for CFCs, HCFCs and/or HFCs, the substitute cryogenic compositions must be effective under the same operating conditions thereby serving as “drop in” replacements for CFCs, HCFCs and/or HFCs or “near drop in” replacements for such materials. In view of this combination of necessary properties, a need still exists for further development of suitable replacement materials, particularly for skin cooling during skin treatments.